The process of thermoforming a decorated sheet to produce a three-dimensional decorated article is well-known in the industry, and inks have been formulated for this process. For example, high elongation inks can be formulated to have sufficient adhesion to the substrate, such as polycarbonate, polyesters, acrylics, and others, and flexibility so that they do not crack during the thermoforming process. The advantages of this process include allowing reverse-printing on a second surface of a transparent plastic substrate so that the visible surface in contact with the environment has resistance to scratch, abrasion, and weathering. Printable conductive inks can further be printed on this surface and back filled with molten plastic to create encapsulated electronic devices. Applications for this process include automotive interior parts such as instrument panels, appliance control panels, moldings, and display moldings. Methods for producing decorated and functionalized thermoformed and in-molded electronic devices have also been described, such as the formation of in-molded capacitive switches and encapsulated displays. In actual practice however, the formation of complex electronic devices which depends on the successful layering of printed graphics, electronic traces, and insulating layers, has been difficult to achieve. What is needed is a set of compatible printable layers, including graphic inks, conductive inks, and insulating or dielectric coatings, which are at once compatible, adhesive, and durable when used to form a complex functional electronic device.
Thermoformable inks have been described. See for example, US 2010/0215918; GB 2359556; and US 2014/0037941. Such thermoformable inks include graphic inks and conductive inks.
Thermorformable electronic devices, and methods for producing these devices have also been described. See, for example, U.S. Pat. No. 8,514,454; US 2012/0314348; U.S. Pat. No. 8,477,506; WO 2011/076717; U.S. Pat. No. 7,486,280; and US 2011/0095090. Such devices include in-molded capacitive switches and touch control panels, control panel assemblies, thermoformed parts for use in devices, and in-molded RFID devices.
The prior art describes various decorative inks for thermoforming and in-molding, and also cases of thermoformable conductive inks. However, the ability to create a complex thermoformed in-molded device comprised of multiple layers of graphic inks, conductive circuitry, and insulating layers, as well as various electronic elements such as displays, lighting, sensors, and the like, has proven difficult to achieve in practice. The formulated conductive inks of this invention can be used to create a high quality IMD or IME part that contains a printed stack of mutually compatible layers that include graphic layers, conductive layers, and dielectric layers. In addition, the layers have similar elongation properties so that cracking and delaminating during the thermoforming process does not occur.
The commercial advantages of an in-mold electronic device with capacitive switch elements rather than mechanical wired devices have been recognized. However, the prior art has not provided a comprehensive system of printable fluids that can work together in a printed stacked array to allow the formation of such complex devices without failures due to poor compatibility of functional layers and other material requirements of thermoforming and in-molding.
A problem to be solved remains to create a system of decorative and functional inks and coatings that can be used together in a layered design to create a decorative and functional part. A conductive layer printed over a decorative layer for example, can have increased resistance because of interference by the underlying decorative layer. Another problem is that a thermoformed system of layered dielectric over a printed conductive circuit undergoes severe cracking due to the incompatibility of the dielectric layer with the printed electronic circuit or due to the inability of the printed circuit to deform without cracking during the thermoforming process. Yet another problem to be solved is maintaining low resistance in a printed circuit which contains a binder system that allows deformation without cracking. In many cases the binder system acts as an insulator for the conductive dispersed phase to such a degree that although the circuit can be formed without cracking, it does not have sufficient conductivity to function reliably in a printed and thermoformed device. A successful solution requires a printable fluid with flexibility during thermoforming, which can maintain conductivity after thermoforming, is compatible with adjacent printed layers, and has excellent intercoat adhesion.